The present invention concerns detection of phase shift modulation in amplitude modulated optical radiation. Such phase shift modulation is employed in laser communications and in laser ranging.
In laser communications systems of this type a laser beam is amplitude modulated by a lower frequency signal, typically in the radio frequency (RF) signal range. Information is imparted to the RF modulation signal by phase shift modulation relative to a stationary signal. The information is extracted at the receiver by demodulation of the phase shift modulation signal.
Laser ranging systems include the so called "three dimensional" laser imaging systems. These systems permit the detection of the range to every picture element of a scene. In such systems the scene to be imaged is illuminated by a laser and the reflected signal is received and detected via a sensor. Typically the laser and the sensor with their accompanying optics are disposed side-by-side or along the same optical axis so that the laser illuminates the same small area of the scene as viewed by the sensor. The scene to be imaged is scanned by moving this combination or by using moving reflectors.
Modulation of the laser source enables range information to be obtained. The laser source is modulated with an RF signal. This modulation signal is extracted from the reflected signal received by the sensor. The modulation signal in the reflected signal is phase shifted from the original modulation signal by an amount equal to the round trip transit time.
In the prior art the received signal is detected via a photosensor. The photosensor output is amplified and the signal demodulated to extract the modulation signal. The extracted modulation signal is compared with the signal of the original modulation source to determine the phase difference. This phase difference corresponds to the information imparted in a communication system. In a three dimensional laser imaging system this phase difference is a measure of the range to the area currently viewed by the sensor. In three dimensional laser imaging systems the modulation frequency is selected so that the range measurements of interest, which may be the relative ranges of a feature rather than the total range, are less than the 360 degree phase ambiguity inherent in this technique.
There is a problem with such a system that causes the accuracy of phase difference detection to be reduced. The magnitude of the received signal can vary by several orders of magnitude. In communications systems this variation is primarily due to variations in range or alignment between the transmitter and the receiver, or due to attenuation along the propagation channel. In three dimensional laser imaging systems the amplitude of the received signal may vary due to regions of differing reflectivity within the scene, or by random interference effects within the reflected radiation which is known as speckle. The electronic amplifier coupled to the photosensor typically has a differing phase shift dependent upon the magnitude of the signal received. Thus the strength of the received signal interferes with the accuracy of the phase difference measurement. This problem is called amplitude-phase crosstalk. There have been previous attempts to reduce this amplitude dependent phase shift in the amplifier or to measure and correct for this crosstalk with limited success.
There is therefore a need for a phase difference detector which has a minimum of amplitude-phase crosstalk.